In forming a flowing sheet of a liquid composition for coating onto a substrate surface, the composition is reshaped from collimated flow in a pipe to sheet flow typically by an apparatus known variously in the art as a die, a distributor, an extruder, a weir, and a hopper. As used herein, all such types of apparatus are referred to collectively as hoppers. A hopper may comprise one or more parallel longitudinal members which are oriented transverse to the direction of liquid flow, which members may be bolted together or otherwise attached to form a hopper unit. A primary member may be referred to as a "hopper body," and one or more secondary members as "hopper bars. " Within a hopper, a flow path for liquid composition typically includes in flow sequence an inlet, one or more transverse distribution voids known as cavities, and a slotted exit from each cavity communicating with either a successive cavity or the exterior of the hopper. The last such slot is commonly known as an exit slot. Alternatively, a hopper distribution apparatus may include a distribution chamber open at the top and having a wall forming a weir for overflow cascade or curtain coating therefrom.
In an extrusion hopper, the downstream end of the exit slot typically defines a coating lip from which the extruded sheet of composition is transferred directly to the passing substrate to be coated. In extrusion/slide hoppers, as are used typically in the manufacture of photographic films and papers, composition is extruded from the exit slot onto an inclined slide surface terminating at a lower edge in a coating lip. The extruded sheet flows down the slide surface under gravity and is transferred to the passing substrate either through a dynamic longitudinal bead, as in bead coating, or a falling curtain, as in curtain coating.
In all such coating methods, the cross-sectional shape and longitudinal uniformity of the lip itself is of paramount importance. Defects in a lip or departures from design resulting from abuse as well as ordinary wear and tear can cause degraded coating uniformity resulting in inferior product or outright waste. Thus it is highly important that hopper coating lips be maintained in a state of near perfection as regards their designed configuration. However, other practical considerations can make this difficult. For example, hoppers are known to have been made from a variety of hardenable stainless steel alloys, such as AISI 304 and 316. For simple hoppers intended for coating only one or a few different compositions simultaneously, the specific gravity of these alloys is not a consideration; but for large hoppers capable of delivering, for example, five or more compositions, the mass of the hopper can become formidable, leading to an alternative choice of lighter alloys comprising, for example, aluminum or titanium. Such alloys can be softer than stainless steel, making the lips of such hoppers more easily damaged and also more easily worn by wiping during cleaning and preparation for coating. When a lip is damaged or worn beyond some useful limit, the hopper must be removed from service and the hopper bar containing the lip must be remachined to recreate the original lip. This reduces the available runtime of the hopper and can be expensive in remachining and reassembly and calibration of the reassembled hopper. Further, the remachined bar is now smaller than previously, which can lead to known problems in recalibration of the hopper. Further, the bar may be remachined only a few times before it is too small for further machining and must be discarded and replaced.
The nominal cross-sectional geometry of a coating hopper lip is an important factor in coating successfully under a specific set of conditions. As conditions are changed, for example, from bead coating to curtain coating, the lip design must also be changed. In the known art, hoppers are not practically interchangeable through various coating conditions or methods because of the extreme difficulty in exchanging the first hopper bar, which carries the coating lip. Instead, typically separate entire hoppers are fabricated for differing coating conditions requiring different hopper lips, at very large capital cost.
Some of the best materials for hopper lips, such as various of the Stellite series of alloys, are impractical for use in building entire hoppers. Such materials can be machined to a virtually perfect lip, and their extreme hardness makes such lips virtually indestructible in normal use. However, many such materials are unsuited for overall hopper manufacture because of high cost, high density, high brittleness, thermal instability, or difficulty in machining.
U.S. Pat. No. 5,639,305 discloses an extrusion hopper having a replaceable, flexible strip forming the overbite edge of the hopper extrusion slot. The strip is held in place either by vacuum applied to the back side of the strip through porting in the hopper bar or by being clamped between two bars. No means is provided for locating and holding the strip precisely within the exit slot to maintain a uniform height of the exit slot and uniform spacing of the edge strip from the substrate being coated. Further, offset between the slot surface and the strip surface is not controlled. FIG. 16 indicates a significant gap at that juncture, which would be unacceptable for a free-flowing slide surface in an extrusion/slide hopper. Also the loss of vacuum could be detrimental to the coating roll hopper and other hardware.
U.S. patent application, Ser. No. 08/826,539, filed Apr. 3, 1997, discloses a metal hopper having a replaceable hopper lip formed from ceramic. The hopper bar and lip element differ substantially in thermal expansion coefficient, and the lip element is shrink fitted to the hopper bar by cooling the ceramic from room temperature to between 0.degree. C. and -196.degree. C., sliding the ceramic lip element over the metal, and allowing the overlap of lip element and bar to compress as the lip element is returned to room temperature. Thus emplaced, the lip element is not readily slidable on the metal bar to relieve thermal stresses, and the system can maintain hopper straightness over a working temperature range only through high elastic modulus of the ceramic.
Thus there is a need for a method and apparatus to allow the replaceable disposition of a specially-formed hopper lip element on a hopper configured to receive such clement wherein the lip element is precisely positioned in a vertical and one horizontal direction while being allowed to slide along the hopper in another horizontal direction to relieve thermal stresses between the hopper and the lip element, thus preventing thermal distortion of the hopper.